Transmission line coupling arrangements



July 10, 1962 K. G. PATTERSON 3,044,026

TRANSMISSION LINE COUPLING ARRANGEMENTS Filed May 26, 1959 \\\T I 5 L L i, L b I \II 1 6\ 2 /a 9\!' 3 I r I fizz Z l I 1 F m n *1 II INVENTOK aw v57; 6L) Pl- Tazson/ FiTToRNsYs tates at ice This invention relates to transmission line coupling arrangements.

More particularly theinvention is concerned with coupling arrangements of the kind in which there are a plurality of coupling sections of transmission line connected between two transmission lines, each coupling section being of uniform characteristic impedance along its length and the electrical distance along each of the two transmission lines between junctions with adjacent pairs of coupling sections and the electrical lengths of the coupling sections all being approximately the same. At

the frequency at which the coupling arrangement is to be used, the electrical length of each of the coupling sections and the spacing of the junctions ofsthe sections with each of the two previously mentioned transmission lines is approximately a quarter wavelength.

A known construction of coupling arrangement of this kind in which the two transmission lines and the coupling sections are waveguides, is described in Section 14.4 of volume 11 of the Massachusetts Institute of Technology Radiation Laboratories Series, which is entitled Technique of Microwave Measurements and is edited by C. G. Montgomery. In Section 14.6 of this book it is stated that it has been usual if there are three or more coupling elements to arrange for the voltage coupling efiected by successive coupling sections to be proportional to the coefiicients of a binomial expansion.

One object of the present invention is to provide a construction of transmission line coupling arrangement which is of the kind specified With three or more coupling sections and which is capable of use over a greater andwidth than a binomial arrangement.

According to the present invention, in a transmission line coupling arrangement of the kind specified in which there are n coupling sections between the two transmission lines, n being greater than two, the characteristic impedance of each coupling section, if the coupling sections are in series connection with the two transmission lines, or the characteristic admittance thereof, if in shunt connection with the two transmission lines, is approximately proportional to sin chug;

where r is the number of the coupling section counted from one end of the arrangement. The invention is particularly advantageous if n is at least four.

One construction of a waveguide coupling arrangement in accordance with the present invention will now be described by way of example with reference to the accompanying drawingin which FIGURES 1 and 2 show cross-sections through the arrangement, FIGURE 1 being a section at the line II in FIGURE 2 and FIGURE 2 being a section at the line IIII in FIGURE 1.

Referring to the drawing, the arrangement provides coupling between two waveguides 1 and 2 which are both of rectangular cross-section, the widths of the broader walls 3 and the narrower walls 4 of each of these waveguides being a and b respectively. Five coupling sections 5 to 9 of waveguide are connected between the two waveguides 1 and 2, each of these sections being straight and of uniform cross-section along its length. The two waveguides 1 and 2 lie with their longitudinal axes parallel to one another and each of the coupling sections 5 to 9 has its longitudinal axis perpendicular to the longitudinal axes of the twowaveguides 1 and 2. At each junction of a coupling section 5, 6, 7, 8 or 9 with one of the waveguides 1 and 2, the coupling section opens into one of the broader walls 3 of the waveguide 1 or 2 and in fact, the opening extends right across that wall of the waveguide. The cross-sectional dimensions of each of the coupling sections of waveguide are thus a and b where b, is the width of the said opening.

Each of the coupling sections 5 to 9 has an electrical length approximately equal to a quarter wavelength at the frequency at which the arrangement is to be used while the spacing of the sections along each of the main waveguides is also approximately a quarter wavelength.

The coupling sections 5 to 9 are in series connection with the two waveguides 1 and 2 and therefore, in accordance with the present invention, the characteristic impedance of the rth coupling section is required to be proportional to The values of this expression for the five values of r are given by the following table:

sin (2r-1)% 0. 3090 0. 8090 1. 0000 0. 3000 The characteristic impedance of each of the coupling sections 5 to9 is directly proportional to the dimension b, of that section and accordingly it is merely necessary for the dimensions b ('=b chosen in the same ratio as the values of sin zr-n set out in the above table.

In one'example of the construction of waveguide coupling arrangement which is described above and which is arranged to split power supplied over either one of the waveguides 1 and 2 equally between both of those waveguides, the dimensions b,. are as follows:

purpose of comparison, it is, therefore, perhaps preferable to consider the ratios of the dimensions h of the coupling sections to 9 in the two cases. These ratios are given below:

. Arrangement Ratio according "Binomial" to the arrangement invention It will be noticed that the ratios bg/ I1 and b /b in the binomial case are not the values that might be expected purely from consideration of the coeflicient of the binomial expansion. The reason for this is that in a binomial coupling arrangement, it is the voltage coupling which is proportional to the coefficients of a binomial expansion, the voltage coupled by each coupling section not being directly proportional to the appropriate dimension i2 of that section.

It the waveguide coupling arrangement described above in accordance with the present invention is for use in the frequency range from 1800 to 2200 megacycles per second, the dimensions a and b may be 4.30 inches and 2.15 inches respectively. The length L of each coupling section is then 1.490 inches while the distance L (=L between the longitudinal axes of the outer pair of coupling sections 5 and 6 is 2.067 inches and the distance L- ('=L between the pair of sections 6 and 7 is 2.107 inches. This coupling arrangement is found to be useful over a broader band of frequencies than the corresponding binomiaF arrangement. In particular the voltage standing wave ratio in each of the waveguides 1 and 2 due to the coupling arrangement under matched termination conditions does not exceed a predetermined value, say 1.05, over a broader band. 7

Although in the embodiment described above, all the coupling sections 5 to 9 have the same physical length, the end sections may preferably be slightly longer so as to give them the desired electrical length when due'allowance is made for the effect of the discontinuities at the junctions of the coupling sections and the waveguides 1 and 2. Similarly these discontinuities give rise to changes in the effective characteristic impedances of the coupling sections 5 to 9, but these effective impedances are in approximately the same ratio as the actual impedances previously discussed herein.

In the coupling arrangement in accordance with the invention which is described above, the coupling sections 5 to 9 are eifectively connected in series with each of the waveguides 1 and 2. Alternatively the coupling sec 4 tions may be connected in shunt with the waveguides, in this case the coupling sections opening into the narrower walls of the waveguides and the characteristic admittances of the coupling sections (instead of the characteristic im-pedances thereof) being related in the manner previously discussed.

It will be understood that the invention is not restricted to waveguide coupling arrangements since it is equally sin (Zr-U where r is the number of the coupling section counted from one end of the arrangement.

2. A transmission line coupling arrangement according to claim 1 wherein n is at least four.

3. A transmission line coupling arrangement according to claim 1 wherein the two transmission lines and the coupling sections are waveguides.

4. A transmission line coupling arrangement of the kind specified in which there are ncoupling sections between two transmission lines, It being greater than two,

wherein the coupling sections are in shunt connection with the two transmission lines and the characteristic admittance of each coupling section is approximately proportional to sin (2% 1% where r is the number of coupling section counted from one end of the arrangement.

5. A transmission line coupling arrangement according to claim 4 wherein the two transmission lines and the coupling sections are waveguides.

References Cited in the file of this patent UNITED STATES PATENTS Sferrazza Jan. 14, 1958 2,866,595 Marie Dec. 30, 1958 

